Dietary whole grains and zinc nutriture

Dear Editor:

In a recent article in the Journal, Vanegas et al. (1) provided 81 adults with whole grains or refined grains in mixed diets for 6 wk. Dietary fiber contents were ∼41 or 21 g/2500 kcal. Outcomes included “a very modest effect on the gut microbiota composition, SCFAs [short-chain fatty acids], and certain indicators of the immune response” in the subjects who consumed whole grains. In our opinion, the report is incomplete: phytate-to-zinc molar ratios of the diets were not disclosed.

Relevant to phytate-to-zinc molar ratios, Hunt et al. (2) conducted a feeding study in 109 adults in which the fiber contents of diets were similar to those in Vanegas et al. (1), in which outcomes were the intestinal absorption of zinc. Each subject received 1 of 10 diets. Five diets provided 21–39 g dietary fiber/2500 kcal daily for 4 wk. In contrast, the other 5 diets provided 18 g dietary fiber/2500 kcal daily for 4 or 8 wk. The respective phytate-to-zinc molar ratios were 15–23 and 2–7. The higher fiber-phytate diet significantly decreased intestinal absorption of ⁶⁵Zn. Plasma zinc concentrations did not reflect the changes in zinc absorption, and no metabolic changes were recognized.

The suppression of intestinal absorption of certain essential metals by dietary phytate has been known since the 1920–1930s. As reviewed (3), in 1961, Prasad and colleagues proposed that zinc deficiency was the cause of growth stunting and hypogonadism in 11 Iranian subsistence farmers who frequently ate clay; later, Prasad led the team who studied 40 stunted hypogonadal Egyptian farmers in a controlled environment. All subsisted on diets based on whole grains and were afflicted by hookworm and schistosomiasis. Zinc treatment and an omnivorous diet facilitated growth and development, in contrast to diet alone. Prasad also found zinc deficiency in 16 subsistence farmers living in 2 desert oases who were not afflicted with hookworm or schistosomiasis. In addition, colleagues in Iran confirmed the efficacy of zinc in stunted hypogondal Iranian subsistence farmers and clarified the role of zinc-binding dietary ligands (e.g., phytate, dietary fibers, lignin) in the illness (3). The practical importance of these findings for US women was suggested by Velie et al. (4). Their case-controlled study found associations between preconceptional maternal nutrition and neural tube defects (NTDs) in 859 mother-infant pairs that included 430 instances of NTDs. The risk of NTDs was inversely associated with maternal preconceptional zinc intake. Also, phytate seemed to modify the zinc-NTD association. The effect of zinc per se was consistent with Meadows et al. (5): fetal growth related directly to the zinc content of maternal and fetal leukocytes and maternal muscle. More recently, Hambidge et al. (6) and Miller et al. (7) confirmed the key role of phytate in human zinc nutriture. Phytate-to-zinc molar ratios ≥12 increase the risk of zinc deficiency, and phytate can account for 88% of the variance in human zinc absorption.

Prasad and colleagues (8, 9) showed immunologic markers to be highly sensitive to zinc status, in contrast to plasma zinc. Of high importance is serum active thymulin, a hormone produced by the thymus. Thymulin mediates the proliferation and differentiation of T-helper cells and the generation of mRNAs of IL-2 and interferon γ (IFN-γ) by T-helper 1 cells. More recently, Zyba et al. (10) reported that low zinc status impairs DNA integrity. We suggest that the indexes reported by Prasad and colleagues (8, 9) and Zyba et al. (10) might be useful in research on the safety of the habitual consumption of whole grains and other seeds, in contrast to foods based on seeds from which phytate and other metal-binding ligands have been removed.